1. Field of the Invention
A chlorine dioxide generator and method for generating chlorine dioxide gas from sodium chlorite through photochemical oxidation.
2. Description of the Prior Art
In order to examine analytical methods for the measurement of chlorine dioxide (ClO.sub.2) in workplace atmospheres, it is necessary to generate a low concentration test gas containing ClO.sub.2. Commonly static test gases have been employed in analysis methods studies. To accomplish this, static test gases of accurately known concentration may be generated employing techniques wherein headspace chlorine dioxide of high concentration is diluted with air in a Tedler gas bag. Unfortunately, the difficulty with employing static ClO.sub.2 test gases is that ClO.sub.2 is very reactive, so that, for example, a test gas containing 0.1 parts-per-million ClO.sub.2 will decompose to other species at a rate of 5 to 20% per hour.
Generally, dynamic test gases are preferred to static test gases for several reasons, and specifically considering reactive species, most dynamic test gas systems show considerably greater concentration stability with time. Because of the very high reactivity of chlorine dioxide, however, standard equipment for generating dynamic test gases, e.g., commercially prepared cylinder gas mixtures and permeation tubes, is not available.
One method for generating dynamic chlorine dioxide test gas mixtures has been evaluated by applicant and found successful: chlorine at low concentrations in air may be bubbled into sodium chlorite solution in a standard laboratory gas washing bottle. If the chlorine concentration is kept low enough, chlorine breakthrough will not occur, and the output of the generator is stable. This method is superior to that of Saltzman (Saltzman, B. E., Analytical Chemistry, 33, 1100, 1961; also Kusnetz, H. L., et al., American Industrial Hygiene Association Journal, 21, 361 (1960)) in that applicant's studies have shown that method, in which chlorine is passed through solid sodium chlorite, to permit chlorine breakthrough. The disadvantage of the method is that chlorine breakthrough is still a possibility, hence test gas monitoring with a simple total oxidant analyzer is not foolproof.
A potentially superior means of generating a dynamic chlorine dioxide test gas has been reported by G. Ishii (Chemical Engineering, Japan, 22 (3), 164 (1958)), in which sodium chlorite in aqueous solution was electrolyzed at the anode of a diaphragm cell, to produce chlorine dioxide which was swept from the liquid into the gas phase. This procedure has not been tested for production of low concentration ClO.sub.2 although test gas dilution is possible. The method suffers the disadvantage of requiring reasonably sophisticated electrochemical equipment for construction of the ClO.sub.2 generator.
Other examples of the prior art are found in "The Photodecomposition of Chlorine Dioxide Solution," Proceedings of the Chemical Society, E. J. Bowen, and W. M. Cheung, 120, 1932; "The Action of Light on Chlorine Dioxide," Proceedings of the Chemical Society, H. Booth and E. J. Bowen, 510, 1925; and in U.S. Pat. Nos. 2,043,284; 2,475,285; 2,683,651; 3,056,270; 3,285,842; 3,429,793; 3,649,493; 3,718,557; and 3,763,006.